Photolytic Mass Loss of Humic Substances Measured with a Quartz Crystal Microbalance

Laboratory studies have shown that photolytic mass loss can be a significant sink for secondary organic aerosol (SOA). Here, we use a quartz crystal microbalance to measure mass loss of Suwannee River Humic Acid (SRHA) and Suwannee River Fulvic Acid (SRFA), surrogates for SOA, exposed to 254, 300, and 405 nm radiation over the course of 24 h. We find that the photolytic mass loss rates of these materials are comparable to those for laboratory-generated limonene and toluene SOA material from the study of Baboomian et al, ACS Earth Space Chem. 2020,4, 1078. Scaling our results to ambient conditions, we estimate that humic substances in aerosols can lose as much as 8% by mass in the first day of exposure in the atmosphere, equivalent to 0.025% of JNO2, the photolysis rate of nitrogen dioxide. By using zero air instead of nitrogen, we also find that the presence of oxygen accelerates the photolytic mass loss rate by a factor of 2 to 4 at all wavelengths suggesting a potential role for reactive oxygen species. UV photolysis of an aqueous SRFA solution demonstrated both photobleaching at UV wavelengths and photoenhancement at visible wavelengths. Ultrahigh-resolution mass spectrometric analysis showed that condensed-phase SRFA photolysis led to decreased intensity in the 100–300 m/z range while aqueous SRFA photolysis resulted in an increase in intensity in the same range. This work reaffirms that photolytic mass loss is a potentially significant sink for SOA, but only on the time scale of a day or two and demonstrates that SRHA and SRFA are suitable surrogates for atmospheric SOA with respect to photolytic mass loss.


Table of Contents
TUV" calculator. 3The UV lamp spectral shape was obtained with a spectroradiometer (RPS-900, International Light Technologies) and then scaled using the azoxybenzene chemical actinometer measurement. 4Details of how the spectrum was measured are shown in Figure S4.The ambient scaling factor was determined by taking ratio between the integration of the photoreactor spectrum over the solar spectrum in the 290 -340 range.This scaling factor is determined as 2.7 in this case.Overall emission spectrum experienced by the cuvette was weighted average of these three measurements ((center*2 + left + right)*0.25).Sensor head pointing up wasn't measured as the cuvette had a plastic stopper which would block direct emission from the top.Finally, the spectral shape obtained was scaled using the chemical actinometer measurement resulting in the scaled spectrum shown in Figure S3.The azoxybenzene actinometer experiment was conducted using an azoxybenzene ethanol solution with 0.2 mM azoxybenzene and 0.6 mM KOH. 3 mL of this solution was added to the cuvette placed in the center of the photo-reactor.During 2 min UV exposure, the UV-Vis absorption spectrum of the solution was collected every 10 s.The azoxybenzene photo-isomerization product has a known molar extinction coefficient at 458 nm of 7600 L mol -1 cm -1 , 4 which is used to convert the absorption to product concentration.The photon flux in the UV range is given by: 5 in which A 0 is the initial azoxybenzene concentration, P is the photo-isomerization product concentration,   is the quantum yield of product formation for this reaction (a constant 0.21 in the UV range), 5 t is the duration of this reaction and I 0 is the photon flux in units of concentration.This equation is linear up to 40% conversion of the product. 5 convert I 0 to F 0 , photon flux in units of photon/cm 2 /s: in which V is the volume of solution (3 mL), N A is Avogadro's number and Area is the area of the cuvette being exposed to light.In our case, Area = 12 cm.Overall, the photon flux inside the chamber was determined to be 3.72 * 10 15 photons/cm 2 /s.This value was then used to scale the spectral shape obtained with the spectroradiometer (Figure S3).The 254 nm lamp and 300 nm LED produced similar artifacts: frequency initially spiked up and then decreased after the light was turned on, and the reverse changes occurred after the light was turned off.Heat generated by the light source was responsible for this type of artifact, as it was a slow process taking more than 10 minutes to stabilize.The 254 nm lamp produced more heat than the 300 nm LED, resulting in a larger frequency shift.This was also confirmed in a heat gun experiment shown in Figure S7.
The 405 nm On-Off artifact differed significantly from the lamp or LED, displaying a rapid frequency spike up or down when the laser was on or off, which suggests a different mechanism compared to the lamp or LED.Kawasaki et al.  (2009) explained this artifact through photo-induced reversible desorption of water molecules from the crystal's gold surface. 1 Since this process is reversible, it does not contribute to mass changes due to photolysis.Similar to the light On-Off experiment (Figure S6), the frequency initially spiked, then decreased with heating, and finally recovered to a higher baseline after the heat was turned off.It is important to note that the heat gun employed in this experiment generated more heat than the light source, resulting in larger magnitude artifacts compared to Figure S6.The observed frequency lag when the heat was turned off can be ascribed to the heat transfer process between the enclosure's exterior and the crystal.The elevated baseline (approximately 12 Hz change) following the On-Off cycle is believed to be due to heat-induced evaporation of the SRFA material.Overall, the artifacts produced by the heat gun were consistent with those observed using the 254/300 nm light source, further corroborating heat as the source of that artifact.Overall, the sample exposed to 254 nm light loses 68.0% of its mass, while the sample exposed to 300 nm light loses 59.0% of its mass.In addition, the majority of these losses occurred early on, with 51% and 30% of the overall mass loss for 254 nm and 300 nm light, respectively, occurring in the first day.SRHA, SRFA data were combined and fit to a single exponential decay.The integral of the product of the exponential and the solar flux over the spectrum yields a fractional mass loss rate under atmospheric conditions, which is 8.25% mass lost over the first day in the atmosphere (during the summer solstice in Athens, GA in 2023).This rate is equivalent to 0.025% of J NO2 calculated for the same 24-hour period, including nighttime hours.The mass spectrum of UV-exposed SRFA sample is plotted with positive intensity (blue) while the mass spectrum of unexposed SRFA is plotted with negative intensity (black) for sake of comparison.Unlike the ESI condensed-phase photolysis experiments described in the main text, the LDI sample was exposed to the light directly on a MALDI plate.The LDI spectra shown here were only filtered by a signal cutoff of 3x10 5 intensity level to remove noise.Formula assignment was not performed as the majority of peaks in LDI-MS cannot be assigned by MFAssignR due to its limitation with multiply-charged peaks.Comparing the irradiated sample to the raw SRFA material, a clear suppression of signals in the 100-300 m/z range was observed, which mirrors the results in the ESI-HR-MS spectra (Figure 7).

Figure S2 .
Figure S2.Control Experiment for QCM baseline drift

Figure S4 .
Figure S4.Demonstration of spectroradiometer measurement in the photoreactor.Emission spectrum inside the UV reactor was measured with a spectroradiometer equipped with a cosine diffusor facing to the left, center, right of the chamber.Overall emission spectrum experienced by the cuvette was weighted average of these three measurements ((center*2 + left + right)*0.25).Sensor head pointing up wasn't measured as the cuvette had a plastic stopper which would block direct emission from the top.Finally, the spectral shape obtained was scaled using the chemical actinometer measurement resulting in the scaled spectrum shown in FigureS3.

Figure S5 .
Figure S5.UV-Vis spectra from the azoxybenzene actinometer experiment.A photo-isomerization product peak with a known molar extinction coefficient is highlighted.

Figure
Figure S6, S7.Control Experiment of light source opening/closing and heating artifacts:

Figure S6 :
Figure S6: Light On-Off response of a bare crystal.A three-hour On-Off cycle (indicated by green and red dotted lines) was conducted with 254/300/405 nm light sources, allowing time between cycles to ensure the baseline stabilized.

Figure S7 :
Figure S7: Heat gun control experiment with SRFA-loaded crystal.A 2-minute On-Off cycle (indicated by green and red dashed lines) was performed using a heat gun to blow hot air into the QCM enclosure.

Figure S9 .
Figure S9.Exponential Fit of Photon Flux Normalized Mass Loss Rate and Solar Spectrum Used for Atmospheric Condition Scaling

Table S1 .
Summary of all QCM experiments in this work *: This run experienced network outage during the first hour after light source on, so no fit was performed.**:Error Bar on the fitted constant was obtained through Python Scipy package curve fit function S5